In liquid infusion processing, the reinforcing fibers are placed within a mold cavity or other mechanism for near-net or net-shape tooling in dry or partially wetted (through the use of a compatible material for adhering the fibers to themselves, i.e. a tackifier) conditions, further wetted with the matrix resin, and cured. Liquid infusion processing may be accomplished by a variety of techniques, including high and low pressure Resin Transfer Molding (RTM), Resin Film Infusion (RFI), Vacuum Assisted Resin Transfer Molding (VARTM) and Same Qualified Resin Transfer Molding (SQRTM).
The liquid infusion process may include any process by which the reinforcing fibers with or without particulate filler material are first placed into a mold cavity, die head, or any other means of near-net or net shaped tooling and then wetted with the resinous matrix, again with or without particulate filler material, and cured.
Ceramic Matrix Composite (CMC) pre-ceramic polymer resins are not chemically stable when heated to temperatures below their pyrolysis temperature and therefore provide a challenge to molding a void/defect free laminate via liquid infusion processes.
Prior to, during and even after initial cure the pre-ceramic resins have demonstrated a propensity for release of gaseous compounds. These gases can interfere with the complete filling of the fiber preform during resin injection, leading to a cured laminate with varying amounts, sizes and shapes of porosity. Unwanted porosity can also be formed for other reasons, such as improper filling of the resins. It is known that during Polymer Infiltration and Pyrolysis (PIP), large pores in the cured laminate will propagate to the pyrolyzed laminate and may remain open within the laminate through final densification.
In addition to the interference of the gases, injection of particulate filled preceramic polymer resin into fiber preforms during RTM can result in filtering of the particulate fillers from the resin, minimizing the beneficial aspects of the addition of the particulate filler material.